This invention relates to a compact, accurate and robust speed sensor for motorcycles, all-terrain vehicles (ATVs), and the like.
Mechanisms for determining the speed of a motorcycle or ATV typically involve sensing the rotational speed of a wheel. The sensed information is conducted to a meter for display, typically as linear velocity in units such as miles per hour.
A preferable location for sensing the wheel speed is near the hub. The main component of the sensor is fixed in place on the fork, for example, or on a stationary (that is, not rotating) bracket to which the brake caliper is fastened. The main component of the sensor is thus positioned to detect the motion of one or more small, magnet “targets” that are carried on the brake rotor and located to pass by the sensor as the wheel rotates.
Space for attaching the stationary component of the sensor is quite limited, so a small sensor is necessary. It is also desirable to configure the sensor so that it remains intact, in place and functional, especially when used on an off-road type of motorcycle or ATV where the sensor may be subjected to potentially damaging elements such as shock, water, mud, brush, etc. In short, it is desirable to use a speed sensor that is compact and robust.
One prior approach to sensing wheel rotation speed provides a hollow, metallic housing within which a reed switch is located. The housing includes external threads and a cylindrical internal bore. The housing may thus be in the form of a hollow bolt that can be threaded into a corresponding threaded aperture near the hub. The reed switch is an electrical switch operated by an applied magnetic field. The switch consists of a pair of contacts on ferrous metal reeds in an airtight glass envelope. The contacts are normally open, making no electrical contact. The switch is actuated (closed) when a magnet that is carried on the brake rotor is moved near the switch. Once the magnet moves past, the reed switch returns to its open state. Leads that extend from the reed switch are connected to wires that are inside of a cable that extends from one end of the housing to a meter or computer on the motorcycle where the vehicle speed is calculated and displayed.
While the reed switch-type speed sensor just described has the attribute of compactness, it is insufficiently reliable, primarily because of the requirement for somewhat delicate moving parts: the thin metal reeds.
Another type of sensor, known as a Hall effect sensor, can be employed for sensing wheel rotational speed. Hall effect sensors include a planar conductor, or Hall plate, along which a current is applied. Variations in the magnetic flux through the plate produce a voltage across the plate that is directly related to the flux density. The magnetic flux density variations can be provided by a magnet that moves past the plate. Alternatively, the Hall plate can be biased with a magnet so that the flux is varied by a ferrous target that moves past the plate through the magnetic field.
Some Hall effect sensors are available in integrated circuit (IC) packages. Such packages are typically planar in shape and have a flat face that must be oriented so that it is generally parallel to, and in close proximity with, the path of the target as it moves across the plate. For convenience, this orientation of the Hall plate relative to the target path will be here referred to as the “transverse-to-target” orientation.
While such Hall effect sensors are generally more reliable than reed switches, they can be made more robust if they include additional protective circuitry for use of the sensor in a severe environment or with an electrical system that is exposed to potentially damaging voltage excursions.
The present invention is directed to a robust Hall effect sensor that includes such protective circuitry and is assembled within a compact housing that can be attached to the wheel in the vicinity of the brake rotor to present the sensor Hall plate in a proper transverse-to-target orientation to detect passing targets, such as the brake rotor mounting bolts.
Other advantages and features of the present invention will become clear upon study of the following portion of this specification and drawings.